Lecture 11 Magnets Earth’s Magnetic Field Magnetic Force on a Current

Fig. 19-1, p.625

Fig. 19-CO, p.624

Magnets Poles of a magnet are the ends where objects are most strongly attracted Two poles, called north and south Like poles repel each other and unlike poles attract each other Similar to electric charges Magnetic poles cannot be isolated If a permanent magnetic is cut in half repeatedly, you will still have a north and a south pole This differs from electric charges There is some theoretical basis for monopoles, but none have been detected

More About Magnetism An unmagnetized piece of iron can be magnetized by stroking it with a magnet Somewhat like stroking an object to charge an object Magnetism can be induced If a piece of iron, for example, is placed near a strong permanent magnet, it will become magnetized

Types of Magnetic Materials Soft magnetic materials, such as iron, are easily magnetized They also tend to lose their magnetism easily Hard magnetic materials, such as cobalt and nickel, are difficult to magnetize They tend to retain their magnetism

Sources of Magnetic Fields The region of space surrounding a moving charge includes a magnetic field The charge will also be surrounded by an electric field A magnetic field surrounds a properly magnetized magnetic material

Magnetic Fields A vector quantity Symbolized by Direction is given by the direction a north pole of a compass needle points in that location Magnetic field lines can be used to show how the field lines, as traced out by a compass, would look

Magnetic Field Lines, sketch A compass can be used to show the direction of the magnetic field lines (a) A sketch of the magnetic field lines (b)

Magnetic Field Lines, Bar Magnet Iron filings are used to show the pattern of the magnetic field lines The direction of the field is the direction a north pole would point

Magnetic Field Lines, Unlike Poles Iron filings are used to show the pattern of the magnetic field lines The direction of the field is the direction a north pole would point Compare to the magnetic field produced by an electric dipole

Magnetic Field Lines, Like Poles Iron filings are used to show the pattern of the electric field lines The direction of the field is the direction a north pole would point Compare to the electric field produced by like charges

Earth’s Magnetic Field The Earth’s geographic north pole corresponds to a magnetic south pole The Earth’s geographic south pole corresponds to a magnetic north pole Strictly speaking, a north pole should be a “north-seeking” pole and a south pole a “south-seeking” pole

Earth’s Magnetic Field The Earth’s magnetic field resembles that achieved by burying a huge bar magnet deep in the Earth’s interior

Fig. 19-4, p.626

Dip Angle of Earth’s Magnetic Field If a compass is free to rotate vertically as well as horizontally, it points to the earth’s surface The angle between the horizontal and the direction of the magnetic field is called the dip angle The farther north the device is moved, the farther from horizontal the compass needle would be The compass needle would be horizontal at the equator and the dip angle would be 0° The compass needle would point straight down at the south magnetic pole and the dip angle would be 90°

More About the Earth’s Magnetic Poles The dip angle of 90° is found at a point just north of Hudson Bay in Canada This is considered to be the location of the south magnetic pole The magnetic and geographic poles are not in the same exact location The difference between true north, at the geographic north pole, and magnetic north is called the magnetic declination The amount of declination varies by location on the earth’s surface

Earth’s Magnetic Declination

Source of the Earth’s Magnetic Field There cannot be large masses of permanently magnetized materials since the high temperatures of the core prevent materials from retaining permanent magnetization The most likely source of the Earth’s magnetic field is believed to be electric currents in the liquid part of the core

Reversals of the Earth’s Magnetic Field The direction of the Earth’s magnetic field reverses every few million years Evidence of these reversals are found in basalts resulting from volcanic activity The origin of the reversals is not understood